frank jorsn frank jorsn - 2 months ago 31
Android Question

Messages size of sensor_msgs.image/compressedImage type will changed when publish/subscriber in ROS-Android?

I need to get the preview-image-data from Android-phone-camera and publish it by ROS, and here is my sample-code:

public void onPreviewFrame(byte[] data, Camera camera) {
if(data != null){
Camera.Size size = camera.getParameters().getPreviewSize();
YuvImage yuvImage = new YuvImage(data, ImageFormat.NV21, size.width, size.height, null);

if(yuvImage != null){
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ChannelBufferOutputStream stream = new ChannelBufferOutputStream(MessageBuffers.dynamicBuffer());

yuvImage.compressToJpeg(new Rect(0, 0, yuvImage.getWidth(), yuvImage.getHeight()), 80, baos);
yuvImage = null;

baos = null;
catch(IOException e){

// compressedImage type
sensor_msgs.CompressedImage compressedImage = compressedImagePublisher.newMessage();
compressedImage.getHeader().setFrameId("xxx"); // frame id

Time curTime = connectedNode.getCurrentTime();
compressedImage.getHeader().setStamp(curTime); // time

compressedImage.setFormat("jpeg"); // format
compressedImage.setData(stream.buffer().copy()); // data

try {
stream = null;
catch (IOException e) {

// publish
System.out.println("-----Publish: " + compressedImage.getData().array().length + "-----");
compressedImage = null;
Log.v("Log_Tag", "-----Failed to get yuvImage!-----");
Log.v("Log_Tag", "-----Failed to get the preview frame!-----");

And then, I had subscribed the topic, just to check if the messages had been published completely and correctly. Just like the following code did:

public void onStart(ConnectedNode node) {
this.connectedNode = node;

// publisher
this.compressedImagePublisher = connectedNode.newPublisher(topic_name, sensor_msgs.CompressedImage._TYPE);
// subscriber
this.compressedImageSubscriber = connectedNode.newSubscriber(topic_name, sensor_msgs.CompressedImage._TYPE);
compressedImageSubscriber.addMessageListener(new MessageListener<CompressedImage>() {
public void onNewMessage(final CompressedImage compressedImage) {
byte[] receivedImageBytes = compressedImage.getData().array();
if(receivedImageBytes != null && receivedImageBytes.length != 0) {
System.out.println("-----Subscribe(+46?): " + receivedImageBytes.length + "-----");

// decode bitmap from byte[] with a strange number of offset and necessary
Bitmap bmp = BitmapFactory.decodeByteArray(receivedImageBytes, offset, receivedImageBytes.length - offset);

I'm so confused about the number of offset. It's means the size of image-bytes had changed after packaged and published by ROS, and if I don't set the offset there're will be wrong to decode a bitmap. And more strangely, sometimes the number of offset had a change too.

I don't know why, and I had read some articles about the jpg structure, and suspect that it's maybe the head-information of the jpg byte messages. However, this problem just happen in ros-android scene.

Anyone have a good idea about this?

OK! I know that the question I asked and problem described previously is terrible, that's why it got 2 negative-points. I'm so sorry about these, and I have to make up my fault by telling you guys more information, and making the problem more clear now.

At first, forget about all the code I pasted before. The problem happened in my ros-android project. In this project, I need to send the sensor messages of compressed image type to ros-server and get the processed image-bytes(jpg format) in publish/subscribe way. And in theory, the size of the image-bytes should be same, and in fact, this had been proved in my ros-C and ros-C# project under same conditions.

However, they're different in ros-android, it's get bigger! For this reason, I can't just decode a bitmap from the image bytes which I had subscribed, and I have to leave out the excrescent bytes with a offset in the image byte array. I don't know why this happened in ros-android or ros-java, and what's these adding part.

I can't find the reason from my code, that's why I pasted my code in detail for you. I really need help! Thanks in advance!


Answer Source

Maybe I really need to check the API first before ask question here. This's a so simple question if I had check the properties of ChannelBuffer in API, cause the property arrayOffset had already told me the answer of this question!